tests: centralize script path setup in conftest.py

Add a pytest-discovered tests/conftest.py for the last30days scripts path and
remove duplicate per-file sys.path.insert boilerplate from tests.

Normalize affected imports to rely on the shared scripts path and remove the
now-unneeded E402 suppressions.

tests/test_competitor_subrun_isolation.py

@@ -1,4 +1,3 @@
-# ruff: noqa: E402
 """Regression tests: main-topic flags must not leak into competitor sub-runs.
 
 Based on 2026-04-22 Kanye West --competitors receipt where Drake and
@@ -10,15 +9,10 @@ via closure capture, config mutation, or any other path.
 from __future__ import annotations
 
 import io
-import sys
 import unittest
 from contextlib import redirect_stderr
-from pathlib import Path
 from unittest import mock
 
-REPO_ROOT = Path(__file__).resolve().parents[1]
-sys.path.insert(0, str(REPO_ROOT / "skills" / "last30days" / "scripts"))
-
 
 def _fake_report(topic: str):
     class _R:
@@ -191,6 +185,5 @@ class SubRunIsolationTests(unittest.TestCase):
             by_topic["Kendrick Lamar"]["config"].get("_auto_resolve_context", ""),
         )
 
-
 if __name__ == "__main__":
     unittest.main()